1. Field of the Invention
The present invention relates generally to the field of mechanical gaseous compressors or vacuum pumps used in industrial, automotive and municipal applications, and more particularly relates to a double rotor multi-lobe type commonly known as rotary lobe type or simply Roots type blowers or vacuum pumps, or known as Roots superchargers in internal combustion engine, and more specifically relates to a shunt pulsation trap for reducing pulsations and induced vibration, noise and harshness (NVH) from such blowers, compressors and pumps for improved efficiency and increased pressure rises.
2. Description of the Prior Art
Rotary lobe blowers or vacuum pumps with potential pressure rises for air or gases up to 30 psig (3:1 ratio) or vacuums up to 28″Hg (15:1 ratio) are widely used in industrial and municipal applications such as power source for loading and unloading bulk materials, for aeration in a waste water treatment plant, for vacuum booster evacuating a container or cleaning municipal sewer lines by vacuum suction. They are also widely used in supercharging automotive engines to boost engine power and could potentially be used for air-conditioning and refrigeration compressors.
Rotary lobe blower (that is: Roots blower or supercharger) or vacuum pump is greatly desired because of their unique compression principle: air or gas is not compressed by conventional positive displacement principle of a fixed volumetric change through the action of a piston, sliding vanes or rotary screw, but instead compressed by a series of waves or shock waves generated by a sudden opening of lobes to blower discharge pressure. The term “shockwave” denotes a physical phenomenon as also occurs in a shock tube where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube. As shown in FIG. 1a-1b, when the diaphragm is broken suddenly, a series of expansion waves is generated propagating from the low-pressure to the high-pressure region at the speed of sound, and simultaneously a shockwave is generated propagating from high-pressure to low-pressure region at a speed faster than the speed of sound.
Roots blower can be also seen as a fast-moving rotary valve and an effective rotary shockwave generator as long as there is a pressure difference between outlet and inlet and rotating speed is fast enough. A stronger shockwave is always associated with a higher pressure difference and faster opening. As illustrated from FIGS. 2a to 2d for a complete cycle of a classical Roots blower, by following one flow cell in a 3-lobe rotary blower, air first enters into spaces between the lobes of a pair of rotors as they are open to the inlet during their outward rotation from inlet to outlet. At lobe position shown in FIG. 2b, the air becomes trapped between two lobes and blower inner casing as it is moved from inlet to outlet, and still no compression and no volume change takes place. As soon as the trapped air is opened to the outlet as shown in FIG. 2c, a series of compression waves or shock waves is produced due to sudden opening to a higher outlet pressure, just like the diaphragm breaking open in a shock tube. The shock wave sweeps through and compresses the trapped air at a speed faster than the speed of sound, and about 5-10 times faster than the rotor tip speed. After the almost instantaneous wave compression, lobes from two rotors meet again, meshing out the compressed air to outlet chamber and return to inlet suction position to start the next cycle, as shown in FIG. 2d. The unique Roots wave compression principle results in a unique performance characteristic: it delivers an almost constant-flow rate at varying pressure or vacuum levels determined by the system back pressure. Moreover, within the wide pressure range, it maintains an almost constant-efficiency no matter what pressure ratio it is operating.
From the above Roots cycle analysis, it should be noted that energy transfers directly between two fluids without using mechanical components like pistons or vaned impellers. Their major benefits are their potential to generate large pressure changes in short time or distance in an efficiency equivalent to those of dry screw compressors. Two rotors of Roots blower are just used as a rotary seal and valve for moving a fixed volume of air from low pressure inlet to high pressure outlet in a fast and continuous manner. In the process, compression is accomplished by waves or shock waves generated by suddenly exposing the fast moving air to higher discharge pressure. In a sense, it is always in an under-compression mode as in the case of conventional positive displacement compressors with a pre-determined pressure ratio of one. Therefore, Roots compression possesses another unique characteristic: it maintains a good efficiency while meeting varying pressure demands. This makes rotary lobe blowers ideal for variable demand applications such as in pneumatic conveying where material clogging can be quickly cleared out or for municipal wastewater treatment aeration tanks where water levels change constantly or for automotive supercharging at different speeds and pressure boosting levels while maintaining a good efficiency throughout the process. Since the compression is achieved through faster moving waves or shock waves without hardware or the associated inertial, rotary lobe blowers can be build very small in size and simple in structure without complicated geometry or rotor contours as other varying volume types, and are capable of a long service life since there are no wearing parts involved for compression.
Despite the above mentioned generally attractive features, several challenges have impeded their extensive commercial applications of the unique Roots wave compression principle. Among them, the number one problem is the pulsation: when pressure waves or shockwaves are generated on low pressure side compressing the air inside lobe cell, a series of expansions waves are generated simultaneously on high pressure side which, together with the reflected pressure wave or shockwaves travel downstream the discharge pipe, creating huge pressure and flow fluctuations that could destroy downstream components, or generate noises as high as 140 dB for high pressure applications. Therefore, a large reactive type pulsation dampener is required at the discharge side of a rotary lobe blower to dampen the air borne pulsations, as shown in FIG. 2e and FIG. 3. The pulsation dampener is generally of long cylindrical shape with large cross sectional area and several chambers in series divided by baffle plates fitted with tubes for reflecting and dampening pulsations. These commercially available dampeners are very effective in pulsation attenuation but suffer additional pressure loss in the process due to high flow velocities induced by waves of large magnitude and due to periodic reversing flows colliding with the main cell flow. Moreover, they themselves generate additional vibrations and noises from their large sheet-metal surfaces typically made from steel weldment. For this reason, the whole blower package is often put into a room-like sound enclosure typically consisting of 5 sided wall panels with sound absorbent material and reflection surfaces, as shown in FIG. 4. Sound enclosure built in this fashion is generally very effective, reducing noise levels by about 20 dB, but it is expensive, bulky, especially not practical for mobile applications. For this reason, Roots compression principles are often cited with mediocre efficiency, very high pulsation and noise, large package size, all of which deter its wider use to more applications in spite of its unique merits out of wave compression.
Various attempts have been made to reduce the air borne pulsations in addition to the conventional method using a serially connected discharge dampener or silencer. One example, as disclosed in U.S. Pat. No. 4,215,977 to Weatherston, is to feed back a portion of the outlet flow through an injection port to the transfer chamber prior to discharge, in an attempt to equalize the cell pressure with the outlet hence reducing the pressure spike when the cell is suddenly exposed to the higher outlet pressure. One of the commercial applications of this technology, for example, is trademarked WhispAir manufactured by Dresser Roots. However, its effectiveness for pulsation attenuation is somehow limited, only achieving 5-10 dB reduction and a discharge dampener silencer is still needed in most of the applications. In theory, having a flow back prior to discharge could reduce pulsation amplitude by elongating releasing time to discharge pressure. But the prior art failed to recognize the existence of finite waves that travel in both directions, hence failed to attenuate them at its source: the waves are simply re-channeled and passed on to the down-stream dampener or silencer without much attenuation. Moreover, the prior art failed to address losses associated with high velocity jet flow through the injection port, compounding the pressure loss already existed from the discharge silencer.
In addition to pulsation problems associated with Roots compression, another often cited limitation is its “inherent mediocre efficiency”, typically ranging from 50-60%, and its low compression ratio (typically 2.2:1, or 18 psig) it can achieve without external cooling. The two factors are somehow tied together resulted from an outlet temperature limit of about 350 F. If efficiency could be higher, say up to 80%, it would dramatically increase the pressure ratio to 3:1, or 30 psig with discharge temperature still at 350 F. It is the low efficiency that hampers Roots compression from being used more widely to higher pressure ratios and more energy sensitive applications like air-conditioning and refrigeration.
One reason for its low efficiency is from extra leakages out of thermally distorted “banana shaped cylinder”. As gas goes from suction port on one side of the cylinder (inner casing) to discharge port on other side of the cylinder with a temperature rise, the discharge side (hot side) of the cylinder will typically bow towards the inlet side (cool side). However, while the blower cylinder is “banana shaped” in hot condition, the rotors remain its original straight shape and relatively uniform temperature, because they continuously experience the cyclic cool and hot air temperature during each rotation. This condition creates an uneven internal clearance at rotor tips and rotor ends between rotor and casing. Some clearance is increased from the cold state, say near discharge side, causing more internal leakage while the other clearance is decreased, posing potential rubbing and seizure failures. The later scenario often forces the design clearances to be set larger than necessary to avoid any potential contact. The result is more leakage flow. The recycled hot leakage gas raises the inlet temperature further more, further increases the discharge temperature. This becomes one of the dominant limiting factors for rotary lobe blower to reach pressure ratio like a dry sliding vane or screw type compressor. Moreover, since blower cylinder is often the structure support for bearing housings located on its sides, the precision bearing alignment in a cold state is thus shifted in a hot condition, causing potential vibrations which in turn inducing more noises.
Accordingly, it is always desirable to provide a new design and construction of a rotary lobe blower that is capable of achieving high pulsation and NVH reduction at source and improving blower efficiency without using an externally connected silencer while being kept light in mass, compact in size and suitable for high efficiency, high pressure ratio applications at the same time.